The present invention relates to an improved construction, of a through-type capacitor, and more particularly to a through-type capacitor having an improved anti-tracking performance suitable as a capacitor for use with a high-frequency filter of the magnetron.
A construction of the through-type capacitor used for the filter of the magnetron is disclosed in JP-U-57-104446 filed for utility model registration in Japan by Tokyo Shibaura Electric Co., Ltd. on Dec. 18, 1980. In the magnetron which is adapted for oscillation with high frequencies, part of the fundamental oscillation electric wave or it's high harmonics is liable to leak outside through power lines. In order to prevent this inconvenience, it is common practice to insert a high-frequency cut-off filter in the line connecting the magnetron and a power supply. The filter is normally configured of a choke coil and a through-type capacitor with an earth electrode thereof connected to a filter case. Since the magnetron has an anode thereof impressed with a voltage of several thousand volts, the same voltage is applied between the central electrode and the earth electrode of the through-type capacitor of the filter.
In the case where the surface of the armor case of insulating material of the through capacitor impressed with such a high voltage is contaminated with dust, salt, carbon, fine metal powder or the like, a leakage current flows on the surface thereof, and the resulting heat generation forms a partial dry zone where the leakage current is cut off. Most of the applied voltage is concentrated to generate a spark discharge at the cut-off portion. The discharge, in turn, carbonizes the surface of the insulating material, which thus forms a conductive path finally to cause a dielectric breakdown. This phenomenon is called "the tracking". The tracking phenomenon is described, for instance, in "Electrical Insulating Materials" pp. 165 to 169, by Tatsunosuke Ashizawa, published by Sangyo Tosho Co., Ltd. in Japan on October 30, 1969. Also, with regard to the insulating material for the armor case of the through-type capacitor, the use of polybutylene terephthalate (PBT) is disclosed in JP-Y2-60-18832, JP-U-63-16427 and JP-U-63-20421 field in Japan by TDK Co., Ltd. on Jan. 19, 1980, July 16, 1986 and July 24, 1986, respectively. The PBT material is flexible and resistant to external shocks, and if used for an armor case of a through-type capacitor, is hard to break while being easily fitted with the earth electrode plate of the capacitor. Further, it has a comparatively high anti-tracking performance.
A tracking is liable to occur if the surface of the through-type capacitor is contaminated during the processes of production or use or if moisture attaches to the surface of the through-type capacitor while being used in an environment subjected to great changes in humidity or temperature. Also, the chance of surface discharge increases with the size reduction of the through-type capacitor which is one of the electronic parts required to be miniaturized. The PBT material which is an organic material, therefore, may develop a tracking if used as an armor case of a through-type capacitor under such environmental conditions as mentioned above. This phenomenon has been confirmed in a test conducted by applying a high voltage periodically between the electrodes of a through-type capacitor in a high-humidity environment.